No. All calculations run in your browser; the numbers you enter are never sent to a server.

Compound Interest Guide: How It Works & the Formula

Q: What is the compound interest formula?

For a lump sum it is A = P(1 + r/n)^(nt), where P is the principal, r is the annual rate as a decimal, n is the number of times interest compounds per year, and t is the number of years. Regular contributions add a future-value-of-an-annuity term on top.

Q: Does compounding frequency really matter?

Yes, but less than people expect. Daily versus monthly compounding at the same nominal rate changes the result only slightly; the rate, the contributions, and the time invested matter far more.

What this tool does

The Compound Interest Calculator takes a starting deposit, a monthly contribution, an interest rate, and a time horizon, then projects your future balance, the total you contributed, the interest earned, a growth chart, and a year-by-year breakdown. It all runs in your browser.

What is compound interest?

Compound interest is interest earned on your interest. Each period, the rate is applied to the whole balance — including the interest already added — so growth accelerates over time. That "snowball" effect is why starting early matters more than almost anything else.

The formula

For a single lump sum, the balance after t years is A = P(1 + r/n)^nt, where P is the principal, r is the annual rate as a decimal, and n is how many times a year interest compounds. When you also add money regularly, the calculator steps month by month: each month it applies interest to the running balance and then adds your contribution.

How to use it

Open the Compound Interest Calculator.
Enter your initial deposit and a monthly contribution (either can be zero).
Set the annual interest rate, the number of years, and the compounding frequency.
Read the future balance, total contributions, and total interest — they update as you type.
Use the chart and the year-by-year table to see when interest starts to outpace contributions.

Does compounding frequency matter?

Less than people expect. At the same nominal rate, daily compounding only edges out monthly by a fraction of a percent. The levers that actually move your result are the rate, how much you contribute, and how long you stay invested.

Your numbers stay private

All calculations happen locally in your browser — nothing you enter is uploaded.

FAQ

What is the compound interest formula?

A = P(1 + r/n)^nt for a lump sum; contributions add a future-value-of-an-annuity term on top.

Does compounding frequency really matter?

Only a little. Rate, contributions, and time invested matter far more.

Is my data uploaded?

No — it all runs in your browser.

This tool gives estimates for general use and is not financial advice.

Ready to try it? Open the Compound Interest Calculator →

Compound interest: how it works